1. Field of the Invention
The present invention relates to a phosphor, and more particularly, to a phosphor having improved luminescence efficiency and a process for preparing the same by uniformly dispersing activators on the surface of a host.
2. Description of the Related Art
In general, FEDs (Field Emission Displays), VFDs (Vacuum Fluorescent Displays), etc. are used as low voltage display devices in which a driving voltage below 1 kV is used. The FED has been widely studied for use as a small flat display device. The VFD is typically used a display device for panels of various home application apparatuses, such as video cassette recorders, microwave ovens, etc., and is also used as a display device for panels of automobiles. In low voltage display devices, the electrons are irradiated from an electron-emitter such as a hot-wire, and the irradiated electrons activate phosphors on the screen of the display device to reproduce graphic images.
Conventionally, the phosphor includes a host, as a main component, and an activator. ZnS, (ZnCd)S, Y.sub.2 O.sub.3, or Y.sub.2 O.sub.2 S are generally used as the host, and various kinds of metals are used as the activator. When ZnS is used as the host, the activators are doped on the host by a thermal diffusion method at 900 to 1000.degree. C. Conversely, when Y.sub.2 O.sub.3 is used as the host, the activators are doped on the host at higher temperatures of 1200 to 1300.degree. C. The activators of the phosphors produced by the above-described conventional methods are uniformly dispersed into the host as shown in FIG. 1. Thus, all activators in the phosphor affect the luminescence of the phosphor only when a high energy electron beam is irradiated to the phosphor in a high voltage display device, such as a cathode ray tube.
In a low voltage display device, an electron beam, which is irradiated from a hot-wire, penetrates into the phosphor by only a few .ANG. from the surface of the host. Therefore, only the activators on the peripheral of the phosphor affect the luminescence efficiency of the phosphor. Therefore, the luminescence of a phosphor of a low voltage display device greatly depends on the activators on the surface of the phosphor. Accordingly, in the case of preparing phosphors for the low voltage display device, if the activators are doped on the host by the conventional thermal diffusion method, the phosphor has poor luminescence due to the dispersion of the activators. Additionally, since the host and the activators are heated at higher temperatures of 900 to 1300.degree. C. to diffuse and crystallize the activators in the host, the production cost of the phosphor may be increased.